The present invention generally relates to an image forming apparatus and, more particularly, to a method of resetting a toner shortage condition and a toner shortage indication of an image forming apparatus.
An electrophotographic copier, facsimile machine, laser printer or similar electrophotographic image forming apparatus is extensively used. Many of such apparatuses have a developing device of the kind using a one-component developer which is constituted by a toner only or a two-component developer which is a mixture of toner and carrier. This kind of developing device has an image carrier in the form of a photoconductive element, a developing roller for developing a latent image electrostatically formed on the image carrier by a toner which is fed to the periphery thereof, and a toner hopper for supplying the toner to the roller. More specifically, in a developing device which uses a two-component developer, a toner hopper is loaded with a toner only while a developing unit is loaded with a mixture of toner and carrier, i.e. a developer. A toner supply roller is interposed between the toner hopper and the developing unit and controlled to supply an adequate amount of toner from the toner hopper to the developing unit. On the other hand, a developing device implemented by a one-component developer has a toner hopper which is formed as a part of a developing unit, because the developer does not contain a carrier.
It is a common practice to provide a developing device of the kind described with an arrangement for determining whether or not an amount of toner great enough to develop a latent image on an image carrier is present. This urges one to supply a fresh toner or to replace a toner cartridge when the toner available in the developing apparatus is short. Various approaches have been proposed in the past for detecting a shortage of toner in the developing device. One of them is to sense the amount of toner existing in the toner hopper in terms of the vibration of toner or the pressure acting on the toner. Another approach relies on the variation in the load acting on an agitator or similar rotatable member which is installed in the toner hopper. Alternatively, the density of a toner image produced on the photoconductive element by development may be sensed. Further, the varying ratio of the toner to the carrier in the developer which exists in the developing unit may be sensed. When it is determined that the toner is short by any of such approaches, the shortage is displayed on an operation board of the apparatus to urge one to supply fresh toner.
The shortage of toner in the developing device is usually displayed in two consecutive stages, i.e., a toner near-end stage and a toner end stage. Specifically, when the sensing arrangement determines that the toner is short, a toner near-end signal is produced to turn on a toner near-end indication on the operation board. In the toner near-end stage, it is possible to produce a predetermined number of more copies, such as fifty more copies, if desired. However, the toner near-end indicator continues to alert a person to the short supply of toner. When the predetermined number of more copies have been produced, a toner end signal apprears to produce a toner end indication on the operation board and, in this stage, any further copying operation is inhibited. Usually, the toner near-end indication and the toner end indication are implemented by the flashing and the continuous glowing of an LED, respectively. In response to the toner near-end or the toner end indication, one may supply fresh toner by opening a part of the toner hopper or, when the developing device is of the type using a toner cartridge, by replacing the empty cartridge with a new cartridge.
In any case, after the supply of fresh toner or the replacement of the toner cartridge, it is necessary to reset (turn off) the toner near-end or the toner end indication. To reset the indication, use may be made of a switch that produces an on/off signal in interlocked relation with the opening and closing of a door included in the image forming apparatus. In the case of an arrangement wherein on the detection of the shortage of toner in a toner cartridge an indicator alerts a person to the short supply of toner, the indicator may be reset as soon as the person replaces the old toner cartridge with a new toner cartridge, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 63-210975.
The switch scheme stated above can surely reset the indication because the switch is interlocked with a door which is necessarily opened and closed to supply fresh toner. However, a problem is that when the door is opened and closed for a certain purpose other than for supplying a toner, the indication is also reset. Specifically, when the door is operated for an unexpected purpose after the toner end indication has been produced to disable the apparatus, the indication will be reset allowing one to regard that the apparatus is ready to operate. Assuming that the apparatus is continuously operated in such a condition, a toner near-end signal appears in due course to turn on the toner near-end indication again and thereby allows extra copies to be produced by the toner remaining in the developing device. If such a resetting operation is repeated a number of times, the developing device itself will soon run out of toner. Should the copying operation be repeated without any toner, reproduced images would have poor density or the carrier existing in the developer alone would deposit on the photoconductive element to cause troubles in the apparatus. On the other hand, a problem with the cartridge replacement scheme is that disproportionately complicated control circuitry is needed for causing an indicator to continuously turn on the indication until the old toner cartridge has been replaced with a new toner cartridge, resulting in an increase in the cost of the apparatus.